1. Field of the Invention
The present invention relates to a high voltage wideband pulse attenuator, and more particularly to a high-voltage wideband pulse attenuator having an attenuation value self-correction function.
2. Discussion of Related Art
Recently, needs for a high-voltage pulse generator having a peak voltage of tens of kV, a FWHM (Full Width Half Maximum) of several nanoseconds or less, and a pulse repetition frequency of several kHz or less has been increasing, resulting in a need for an apparatus for measuring an output waveform of such a high-voltage pulse generator. However, a probe provided by a conventional high-speed wideband oscilloscope has a limitation in measuring an output waveform of a high-voltage nanosecond pulse generator, and thus an attenuator for attenuating a pulse signal is required.
A high-voltage wideband pulse signal has a time-limited characteristic that a peak voltage of a pulse and a pulse width are tens of kV and several ns or less in a time domain, and a characteristic of an unlimited spectrum in a frequency domain. To attenuate such a high-voltage wideband pulse signal, both of the following two conditions should be satisfied.
First, an attenuator should have an impedance matching characteristic at least in a first null bandwidth BWFirst-null of Equation 1 below. Needless to say, an ideal pulse attenuator would achieve impedance matching in a whole frequency band, which is impossible to realize. Thus, by satisfying the impedance matching characteristic at least in the first null bandwidth, it is possible to prevent a pulse generator from being deteriorated by a reflected pulse.
                              BW                      First            -            null                          =                  1                      t            r                                              [                  Equation          ⁢                                          ⁢          1                ]            
Here, tr denotes a rise time.
Second, insulation performance for a high-voltage wideband pulse signal should be satisfied. In other words, an attenuator should be able to prevent insulation breakdown from being caused by a high-voltage wideband pulse signal.
However, there is a trade-off relationship between the two conditions, and conventional attenuators cannot satisfy both of the two conditions. More specifically, to obtain an excellent frequency characteristic, a resistance unit of an attenuator circuit should be physically so small that a resistance characteristic is not lost due to stray inductance or stray capacitance. On the other hand, to attenuate a high-voltage wideband pulse signal without insulation breakdown, the interval between electrodes of the resistance unit should be large. As a result, when the interval between electrodes of the resistance unit is increased to prevent insulation breakdown, the frequency characteristic of the attenuator deteriorates.
Meanwhile, the resistance unit used in the attenuator circuit for attenuating a high-voltage wideband pulse signal may be exposed to high energy, and properties of the resistance unit may be changed. Thus, it is necessary to examine characteristics of the resistance unit before and after a high voltage pulse test. However, conventional measurement of characteristics of the resistance unit requires other test assisting devices, cables, etc., and thus is inconvenient.
A structure and problem of a conventional pulse attenuator and a conventional capacitive divider circuit will be described in detail below.
FIG. 1 is a cross-sectional view of a conventional T-shaped resistive attenuator.
As shown in the drawing, a conventional T-shaped resistance attenuator 200 has a coaxial structure employing a stick resistor R made of a combination of a ceramic material and a metallic film material. Since the resistor R has a long physical length, it is not regarded as a lumped element at GHz frequency band. Thus, by exponentially reducing a coaxial external diameter, stray inductance and stray capacitance of the stick resistor R cancel each other, so that the stick resistor R can operate as a resistor. Here, the stick resistor R does not have small resistance and has a large breakdown voltage for a high-voltage pulse. Thus, the stick resistor R is useful in attenuating a high-voltage signal.
However, it is difficult to insulate a central electrode from the T-shaped stick resistor R. More specifically, a breakdown voltage of each unit length (mm) differs greatly according to the dielectric quality of a coaxial line, but a breakdown voltage of a dielectric surface is several kV or less per millimeter (mm). When the T-shaped central electrode and an oval case grounding structure 10 are close to each other, insulation breakdown is occurred by an incident high-voltage pulse of tens of kV or more along the dielectric surface. Thus, the T-shaped resistive attenuator 100 is not appropriate for attenuating a high-voltage pulse of tens of kV.
FIG. 2 is a cross-sectional view of a conventional capacitive divider.
As shown in the drawing, in a conventional capacitive divider 200, a U-shaped electrode 20 is inserted between a pulse output line and the ground to implement a pulse divider circuit. Herein, the pulse divider circuit divide voltage by a series structure of a capacitance C1 formed between a ground 21 and the U-shaped electrode 20 and a capacitance C2 formed between a coaxial line 23 and a U-shaped electrode 20. Thus, an impulse output of hundreds of kV in a pulse forming line of an intense electron beam accelerator can be measured with division by several thousands.
In particular, the capacitance C1 formed between the U-shaped electrode 20 and the ground 21 is made to have a value several hundred times to several thousand times that of the capacitance C2 formed between the coaxial line 23 and the U-shaped electrode 20, thereby maintaining an overall capacitance connected in series from the coaxial line 23. Thus, the capacitive divider 200 can be implemented to have a large division ratio without affecting coaxial line characteristic impedance.
However, the conventional capacitive divider is only used to monitor a high-voltage pulse signal in a coupling method, and has a limitation in monitoring a pulse state in real time while attenuating a pulse.
Also, to specify a pulse signal, a test assisting device, a cable, etc. are required. In particular, since an additional capacitive divider should be used, coaxial impedance becomes discontinuous.
Further, to provide a pulse output attenuated to several decibels, the U-shaped electrode 20 should be disposed so close to a coaxial electrode of an output unit in the structure of the conventional capacitive divider that the capacitance C1 formed between the U-shaped electrode 20 and the ground 21 has a similar value to that of the capacitance C2 formed between the coaxial line 23 and the U-shaped electrode 20. However, as the U-shaped electrode 20 and the coaxial electrode of the output unit approach each other, combined capacitance decreases, and output impedance cannot be maintained for 50-ohm.